J. CUXNINGHAM
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taining the secondary amino group has been reestablished. This would require the NH stretching band to appear again, an expectation which is fulfilled in the infrared spectrum of 11. Chemical proof for the correctness of assigning compound
Yol. 65
I1 the structure of 2,5-diphenylpyrrole-3-diazonium chloride is its capability of undergoing coupling reactions with suitable coupling components. Diazo coupling with other pyrrole derivatives has thus led to the new compound class of a~opyrroles.~
RADIATION CHEMISTRY OF IONIC SOLIDS. I. DIFFUSIONCONTROLLED MECHANISM FOR RADIOLYSIS OF IONIC NITRBTESl BY J. CUNNINGHAM Argonne National Laboratory, Argonne, Illinois Received September $9, 1960
The G N Ovalues ~ for the radiolysis of alkali-metal nitrates have been determined at temperatures from -110" to 340'. The accuracy of the method was adequate to definitely detect a kinetic isotope effect in the rate of radiolysis of IIN140318 relative to KN140316 at 25'. The ratio of the G-values of these isotopic KN03 materials was 1.03 f 0.03 a t -110" but 1.12 It 0.02 a t 25". A model which identifies the rate-determining process in the radiolysis of the various nitrates as the jump probability for escape of the oxygen fragment from an excited Nos-, gives consistent results when treated by a theory developed for diffusion in metals.
Several factors of possible importance in the mechanisms of radiation-induced decomposition of ionic solids arise from their rigidity, order and defect structure. These include : (1) greatly decreased probability of rapid diffusion of product species away from the reaction sites; (2) ease of propagation of exciton waves through the ordered lattices and the existence of stable trapping centers for excitons and electrons; (3) Existence of preferred directions for dissociation and product orientation. The relative importance of these factors is being studied in various solids. The alkali-metal nitrates form the first sequence of salts investigated. Large differences have been reported between the rates of radiation-induced decomposition of various solid nitrates under either ultraviolet12 y-ray,3 or X-ray4 irradiation. While there is general agreement that the primary process is unimolecular dissociation of an NO1 species producing nitrite and oxygen,z-7 the varying sensitivities have been attributed to (A) different polarizing power of the cations12 (B) to competition between nitrate ions and oxygen atoms for the oxygen fragments produced13and (C) to differences in the closeness of packing of the crystal structure^.^ Factor (C) is related to ease of diffusion and the present paper reports further investigations of the importance of this "cage" effect in the 7-ray radiolysis of alkalimetal nitrates.
Experimental Holders.-Samples for irradiation were weighed to f0.005 mg. into numbered Pyrex holders( H) accurately ground to 0.455 f 0.025 cm. 0.d. (See Fig. 1.) Care was taken that samples always occupied -2 mm. length a t the bottom of the 1 mm. bore of the holders. They were com(1) Based on work performed under the auspices of the U. S. Atomic Enerpy Cornmiasion. (2) P. Doigan and T. W. Davis, J. Ph,s. Chem., 56, 764 (1952). (3) C. J. Hoehanadel and T. W. Davis, J . Chem. Phvs., 27, 333 (1957). (4) J. Cunningham and H. G. Ileal, Trans. Faraday Soc., 64, 1355 (1958). (5) A. 0. Allen and J. A . Ghormley, J . Chem. Phys., 15, 208 (1817). (6) L. K. Narayanswamg, Trans. Faradag SOC., 31, 1411 (1935). (7) G. Hennig, R. Lees and M. S. Matheson, J . Chem. Phys., 2 1 664 (1953).
pacted down and stray grains removed from the inside walls with a polished tungsten rod. Irradiation Vessel.-The principal features of the stainless steel vessel, designed to give exactly reproducible positioning of the sample holders, are diagrammatically represented in Fig. 1. The holders were pressed tightly into numbered slots in the copper block A by retaining bands. Their vertical positioning was made exact by the heatradiation shield s, and lateral changes in the position of A relative to D were prevented by four clamping screws, F. The 4500 curie Corn y r a y source rod seated reproducibly into cup D, and the outer vessel E seated immovable into holes in a fixed stand in the radiation chamber. Temperature Control.-Temperatures of 25 and - 110" were registered by thermocouples embedded in sample holders a t normal positions during irradiation, when tap water or liquid N2 was supplied to can B. Pressure was normally lo-^ mm. under continuous pumping during irradiation. Temperatures up to 350' were obtained using cartridge heaters embedded in A. Dosimetry.-Absolute dosimetry a t the sample positions was difficult because of the small sample-holder volumes. Reproducible values were obtained with FeS04/CuSOp dosimeter solutions* by combining, for each reading of optical density, equal aliquots from 4 quartz holders of i.d. 0.3 em. and 0.d. 0.455 i 0.025 em. They agreed to tvithin 6% with a dose rate of 2.7 X 1021e.v./l./min. determined by the Fricke dosimeter at the same distance as the samples. Absorbed doses were calculated using, true mass-absorption coefficients reported in the literature,a the above value of flux, values of the relative intensity a t each position determined by KNOa,and correction factors for source decay. Internal Dosimetry.-Several calibration runs were made over the duration of the experiments with KN03 in all 16 positions. At 25 and -110' the reproducibility of the relative decompositions at the various positions in any run was better than &I%. The decomposition at the referenrtb positions in several standard runs a t 25" did not differ by more than 2% from that calculated from the decay of source intensitv, and the average difference was less than lVp. In all irradiations several normal KN03 samples were used to check that the relative intensities did not change and to check the actual dose rate. Only above 130' was it found necessary to apply a small correction for the intensity in the outer positions due to expansion of the copper block. Preparation of Samples.-All normal nitrates n-ere rzcrystallized and the middle fractions dried in vacuo a t 120 They were ground in an agate mortar but not sieved. Nitrates with high enrichment in stable isotopes W 5and were obtained from Isomet Gorp. and thr T17eizmannInstitute Israel, respectively. KNW3 and N a Y O a in the text refer to materials supplied as being 99.6% enriched i n 1 1 6 , and II. Bass a n d H. P. Broida, edltors, "Formation and Trapping of Free Radicals." Academic Press, New York, N. Y., 1960. (24) D. Lazarus, Paper to be presented a t IAEA Conference Sept. 1960. (26) D. Lazarus, private communication, 1960. (26) G. M. McCracken a n d H. M. Love, Phya. Rev. Letters, 6 , 201 (1960). (27) H. C. Urey, J . Chem. Soc.. 562 (1947).
April, 1961
DIFFVSIOSCOSTROLLED AIECHSXISM FOR RADIOLYSIS OF IOSICKITRATES
lattice cage would obviously cease at the melting point. The similarity of the G N O ~values for the molten salts where the dissociating excited species is surrounded in all nitrates by a relatively easily ruptured liquid envelope agrees well with the behavior expected from the The initial diflerence of 12% in the rate of radiolysis of KK1401803 apparently disappeared above the “break” at 0.015 fraction decomposed. A previous explanation of the KK03 “break” attributed it to the build-up of oxygen molecules interstitially in the lattice and consequent 30% reduction of probability of dissociation due to decreased “free space.” The result above conflicts with this simple explanation, since a substantially unaltered isotope effect would be predicted by it. Recent results by Johnson and Forten2* have shown that the change in slope of the kinetic plot for KN03 IS accompanied by: (a) sharp increase in the heat of solution (H) of the irradiated material. The plot of H vs. dose shows a sudden sharp rise just before that dose corresponding to the “break” and these anomalously high H-values are obtained only for a small yange of doses, after which the plot regains a slow gradual increase slightly greater than that before the anomalous rise. Samples having the anomalously high H-values regained normal values when annealed at 150”; (b) an abrupt decrease in density of -1% with density values constant to with 0.1% before and after this change. These workers associate the additional H to strain energy of the lattice generated by the misfit of the oxygeu molecules, and possibly nitrite. They also found additional H for irradiated KaN03 and CsS O 3 at the same decompositions a t which inflections occurred in the kinetic plots. In agreement with the ideas here developed for the relevance of free-space to the diffusion of oxygen, the additional strain energy was found to be five times greater for ?jaNOQ, than for KN03 which has a much larger free-space. John son discusses evidence favoring the view that this strain energy causes the change to a crystal lattice of more open structure in which increased vibration-rotation of the nitrate ions occurs. The decrease in Gyo2- value is attributed to increased vibration-rotation of the nitrate ions in the new structure resulting in added dissipation of energy.28 Since this postulate would not change the nature of the dissociation event, it does not appear probable that it can account for the disappear(28) E. R. Johnson and J. Foiten, Prog. Rept., June 1960, BEC AT(30 1 1824)
635
ance of the isotope effect. There seems little doubt, in view of Johnson’s results, and the persistence to high decomposition of a large isotope effect when the oxygen escapes continuously from the crystal under irradiation at 122”. (Table II), that oxygen molecules are intimately involved in the occurrence of the “break.” Furthermore, Johnson’s data indicate that interstitial molecules are not present in large numbers above the break because the anomalously high H attributed to their strain energy does not persist after the inflection. There is other evidence that the oxygen is present in gas pockets at pressures up to 1400 atmospheres. Participation of the gas in these pockets in the reaction path could occur by any of several mechanisms, but the evidence available is insufficient to decide between them. To be acceptable any mechanism must (a) account for the absence of any large isotope effect, (b) explain why the inhibiting effect in KN03 apparently does not occur til the lattice expands, and (c) why the decomposition continues to be 1st order in [nitrate] above the break. While reaction schemes involving the participation of gas pockets in the dissociative event can account for (a) by replacing the isotopically sensitive diffusion-controlled dissociative event by an equilibrium-type step, possibly with ozone molecules as intermediates, it does not appear profitable a t this stage to advance unsupported mechanisms for the dissociation above the break. Experiments are in preparation to study the nature of the occluded oxygen product by measurements of static magnetic susceptibility, which may provide the information needed to clarify the mechanism in this region. Conclusions The representation of the dissociation of nitrate ions in terms of a diffusion-based model gives a satisfactory and consistent account of the relationship of the free-space to the rate of dissociation of the alkali metal nitrates a t high and low temperatures. The kinetic isotope effects observed, which are anomalous on normal theories of isotope effects, are also accounted for by this model, but only for decompositions below the inflection in the kinetic plots. Further information is needed to clarify the nature of the dissociation process at higher decomposition. Acknowledgment.-The author wishes to thank ,S. Hrobar for her assistance with the many colorimetric analyses for nitrite, and is indebted to Dr. 11.Matheson and Dr. G. Montet for h e l ~ f u l discussion.
